As shown in FIG. 1, in the existing carrier superposition technology, FPGA (field programmable gate array) delivers the following scaled power signal to the DAC (digital-to-analog converter): −(Full Scale) XdB (i.e. XdBFS), and the power of the baseband signal from BBU (baseband processing unit) is −XdBFS, and the gain of DUC (digital up conversion), CFR (crest factor reduction), and DPD (digital pre-distortion) is ZdB. Baseband data length shall be considered as 16 bits. In TD-LTE (Time Division Long Term Evolution) network, considering TD-LTE has high peak-to-average ratio (PAR), small number of carriers, and wide signal band, thus the signal power of it is considered to be −XdBFS.
The peak to average ratio (PAR) of the signal after DUC/CFR is about TdB. The peak amplitude of the output signal is boosted by the DPI) in order to correct the peak distortion of the amplifier. Based on the improvement of the DPD algorithm, the output power of the amplifier can be made to contain P-3 dB, which can increase the peak power of the signal 3 dB after DPD, and keep the average power unchanged. The scaled amplitude of the maximum transmitter output power FPGA to DAC interface is −XdBES.
As more carriers are superposed, as shown in FIG. 2 of multi-carrier superposition, system like LTE-ADVANCED shall support the combination of up to five 20 MHz LTE carriers, where the signal peak-to-average ratio (PAR) probably shall increase. After multi-carrier superposition, peak truncation is usually performed. Before digital superposition, it should be ensured that the power of all the superposed signals is equal to the scaled value. Although DUC/CFR does not introduce gain variation, but as the number of carriers increases, the PAR of the signal may exceed the scaling value −XdBFS, which can cause signal overflow even before CFR, thus undermining signal quality. Since this device does not have saturation processing, it will soon damage the out-of-band signal quality Adjacent Channel Power Ratio (ACPR) and the in-band signal quality Error Vector Magnitude (EVM). Since this method of direct truncation at the peak amplitude shall cause the biggest signal fluctuation, which may cause these signals change from maximum value to minimum value, or change of signal sign, thus leading to signal mutation, which consequently affect EVM and ACPR of the signal.
FIG. 3 is the diagram of multi-carrier superposition overflow determination process in the existing technology which does not have saturation processing. It can be seen that, as the number of carrier increases, the peak-to-average ratio of the signal will be higher and higher, and the superposed carrier signal can easily overflow. In this case, if each carrier is scaled too low, the accuracy will be lost; yet if the scaling is reasonable, the multi-carrier superposition will cause signal overflow. Therefore, how to solve the problem of signal overflow after multi-carrier superposition in the existing technology becomes a technical problem urgently need to be solved by professionals in this field.